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[Bug 1877794] Re: Constant Folding on 64-bit Subtraction causes SIGILL o
From: |
Catherine A. Frederick |
Subject: |
[Bug 1877794] Re: Constant Folding on 64-bit Subtraction causes SIGILL on linux-user glxgears ppc64le to x86_64 by way of generating bad shift instruction with c=-1 |
Date: |
Sun, 10 May 2020 18:38:05 -0000 |
I'm on 5.0-rc4 add a few patches implementing a subset of drm/amdgpu
support, with mesa-utils 8.4.0-1. Important note I guess: I can't get
the crash to trigger under llvmpipe/softrast, I can only get it on
RadeonSI. I valgrind'd qemu only to not find anything on the host-side.
I'm 99% sure this isn't a memory corruption bug from my patches, anyway,
and it's not replicable on upstream qemu because DRM simply isn't
functional. (Oh dear, I think I might be on my own until I can get this
stuff upstreamed.)
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https://bugs.launchpad.net/bugs/1877794
Title:
Constant Folding on 64-bit Subtraction causes SIGILL on linux-user
glxgears ppc64le to x86_64 by way of generating bad shift instruction
with c=-1
Status in QEMU:
New
Bug description:
Hello, I've been recently working on my own little branch of QEMU
implementing the drm IOCTLs, when I discovered that glxgears seems to
crash in GLXSwapBuffers(); with a SIGILL. I investigated this for
about 2 weeks, manually trying to trace the call stack, only to find
that we seemingly crash in a bad shift instruction. Originally
intended to be an shr_i64 generated to an RLDICL, we end up with an
all ones(-1) c value, which gets thrown to the macro for generating
the MB, and replaces the instruction with mostly ones. This new
instruction, FFFFFFE0 is invalid on ppc64le, and crashes in a host
SIGILL in codegen_buffer. I tried to see if the output of translate.c
had this bad instruction, but all I got were two (shr eax, cl)
instructions, and upon creating a test program with shr (eax, cl) in
it, nothing happened. Then figuring that there was nothing actually
wrong with the instruction in the first place, I turned my eye to the
optimizer, and completely disabled constant folding for arithmetic
instructions. This seemed to actually resolve the issue, and then I
slowly enabled constant folding again on various instructions only to
find that enabling not on the shifts, but on subtraction seemed to
cause the bug to reappear. I am bewildered and frankly at this point
I'm not sure I have a chance in hell of figuring out what causes it,
so I'm throwing it here.
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